Some types of light sources that operate by line power and that illuminate a scene produce a brightness that varies sinusoidally according to the square of the line voltage. Thus, these light sources flicker at twice the frequency of the line power. Halogen and incandescent lamps are typical examples.
Usually, the brightness of scenes illuminated by such sources is low, and photography typically requires relatively long exposure times, exceeding the flicker period. Under these conditions, the images shot with an electronic rolling shutter sensor reveal few visual artifacts. To reduce any artifact in these conditions, some sensors are programmed so that the exposure time is a multiple of the flicker period.
FIGS. 1a and 1b show a high brightness situation under illumination by a flickering source, whereby an exposure time smaller than the flicker period may be desirable. A similar situation may occur when the sensor is very sensitive. The brightness P is shown in the upper part of the figure by a sinusoid of period Tf.
In a rolling shutter array sensor, the rolling shutter effect is obtained by the fact that the pixels of the array are operated one row after the other. Thus, the exposure interval of each row is offset relative to the exposure interval of the previous row, by a time equal to the scan period Tr of the array, or frame period, divided by the number of rows of the array.
FIG. 1a shows an example of succession of exposure intervals for four consecutive rows R1 to R4. The intervals are of a same duration and offset with respect to each other by a period Tr. In each interval, a different portion of a period of the sinusoid representing the brightness is integrated. The result is that successive rows integrate different levels of brightness.
FIG. 1b illustrates a portion of an image rendered in this situation for a uniform gray scene. The rendered image, whereas it should be uniform, includes alternating dark and light bands whose number is equal to the ratio between the frame period and the flicker period.
Some rolling shutter image sensors, such as disclosed in U.S. Pat. No. 8,253,090, are capable of multiple integrations (or exposures) per frame. This type of sensor is used, for example, to produce several versions of the same image with different exposure levels. These versions can then be combined to produce a high dynamic range or HDR image.
FIG. 2 illustrates an exemplary operation of a sensor having three integration intervals used in a high brightness environment under flickering light. A same row is subjected to three consecutive integrations, the first over a short time interval Tia, the second over a medium time interval Tib, for example the double of interval Tia, and the third over a long time interval Tic, for example four times longer than interval Tia. This produces three images, respectively dark, medium, and bright. The rows of values obtained at the end of the integration intervals are combined to produce the HDR image.
The values may be combined in different known manners, for example using an algorithm designed to bring out detail in overexposed areas and underexposed areas. For this purpose, the algorithm may analyze each pixel of the medium image, and replace it with a weighted pixel of the bright image or of the dark image, depending on whether the analyzed pixel has a brightness level below a dark threshold or above a bright threshold.
In the presence of bright flickering light, the integration intervals may, as shown, be shorter than the flicker period. In this case, a situation similar to that of FIG. 1a is encountered, where the integration intervals for each row see a different portion of a period of the sinusoid representing the illumination brightness. The resulting image also has alternating light and dark bands.